This invention relates to a solid latent polymeric initiator. This initiator is useful for one-part cure-on-demand systems.
It is well known that the polymerization of oxiranes and similar strained heterocyclic groups can be initiated by nucleophiles, such as nitrogen-containing nucleophiles. For example, imidazole and its substituted derivatives have been investigated as initiators (i.e., curing agents) for epoxy resins and have been found to produce cured resins with good properties. (See for example, Paul F. Bruins, Ed., "Epoxy Resin Technology", Interscience Publishers, New York (1968), incorporated herein by reference.)
Ordinarily, imidazoles are not considered latent initiators because the unshared electron pair on the nitrogen groups are quite reactive with oxiranes even at ambient temperatures. Therefore, it has been found necessary to complex or encapsulate imidazoles to provide latency. Examples of latency provided by complexation can be found in German Offenlegungsschrift 1,904,641 (published Nov. 6, 1969, incorporated herein by reference), which discloses a class of curing agents obtained by reacting imidazoles with metal salts to form metal salt complexes of imidazoles; and U.S. Pat. No. 4,101,514, incorporated herein by reference, which describes metal perfluoralkylsulfonateimidazole type latent curatives which controllably initiate ring-opening reactions of a strained heterocyclic ring.
U.S. Pat. No. 3,018,258 (incorporated herein by reference) discloses coating epoxide-reactive materials with an inactive gelatin. U.S. Pat. Nos. 4,225,460 and 4,503,161 (incorporated herein by reference), disclose microencapsulating Lewis acid initiators with various inert polymers.
There remains a need for inexpensive, easily prepared latent initiators which provide rapid cure-on-demand one-part adhesives with improved shelf stability.